Novel murine recombinant murine leukemia viruses (MuLV's) were isolated from naturally occurring and virus-induced murine leukemias. Selective host systems were derived. De novo recombination was detected in ecotropic MuLV-induced leukemia. Recombinant envelope glycoprotein (gp)70 molecules were found in virus-free lymphomas. New models of leukemogenesis were developed. Seroimmunoprophylaxis with anti-MuLV gp70 IgG was successful in AKR mice. Virus release was eliminated. Breeding of protected AKR mice yielded sequential generaitons of virus-negative AKR mice.